1. Field of the Invention
The present invention is broadly concerned with high density, multiple stage, single pass rotary drum dryers especially useful for the high-efficiency drying of moisture-laden products. More particularly, the invention is concerned with such dryers which include an initial, primarily convection drying stage, a final, primarily conductive drying stage, and an intermediate multiple-zone stage where both convective and conductive drying occurs; the individual zones within the intermediate stage are equipped with internal flighting designed so that the heat transfer ratio (the total zone heat transferring surface area divided by the zone volume) progressively and substantially uniformly increases along the length of the intermediate stage.
The single pass drying apparatus is especially useful for efficiently removing moisture from various products such as materials having significant protein and fat contents without deleterious effects on these constituents. It is known that the effectiveness of a heat exchanger is defined by the difference between the inlet temperature and the outlet temperature. More efficient drying is accomplished with the present invention than prior single pass dryers because the apparatus permits higher than conventional air flow velocities while providing an improved xcex94T difference between inlet and outlet temperatures.
2. Description of the Prior Art
Drying of large volumes of fragmented fibrous materials has long been carried out in heat exchangers consisting of one or more elongated, generally horizontally oriented drums. Hot gases are caused to flow through each to remove moisture from the material by heat exchange between the hot gases and the fibrous product. Generally speaking, a burner is disposed to direct hot products of combustion directly into the inlet of the drum which also receives the moisture-bearing material to be dried. However, advantage has also been taken of other sources of waste heat. After removal of the requisite amount of moisture from the material, the dried product is directed into a collector or other receiving means at the outlet of the heat exchange drum. A blower or equivalent device is provided to accomplish the required rate of flow of hot gases through the drum heat exchanger.
Three pass dryers have been used in the past which include a single rotatable drum with concentric stages arranged so that the material being dried traverses the drum in a serpentine fashion. Three pass dryers are relatively expensive but have been used primarily because of the decreased product residence time necessary to obtain adequate drying, while minimizing ground space in the drying plant. A limiting factor in the use of three pass dryers has been the restricted inlet opening of the concentrically arranged drying zones, thus resulting in a fairly severe heat transfer in the first pass. High temperatures have been tolerated in the first pass of the three pass dryers in connection with the drying of alfalfa because the product typically is introduced into the three pass dryer at a moisture level of about 80%. The latent heat transfer that occurs in the first pass thereby protects the product notwithstanding the high temperature level that exists in the first pass drying zone.
In the case of prior single pass dryers, efforts to increase the air flow velocity simply resulted in excessive blowing of the material out of the dryer and resulting inadequate product retention time. A by-product of the decreased retention time was a lessening of the xcex94T between the inlet and outlet temperatures of the dryer. Even at air velocities of no more than about 500 feet per minute, the resulting discharge temperature on most products was found to be in the range of 300xc2x0 F. to 350xc2x0 F.
Single pass dryers, as contrasted with three pass dryers, are particularly useful for drying temperature-sensitive products that either have a substantially lower initial moisture content than relatively wet alfalfa, as for example about 30%, or that are blended with previously dried material to bring the moisture content of the product entering the inlet of the dryer to about that moisture level. The single pass dryer may be operated at a substantially higher throughput than a three pass dryer. In addition, high temperature levels in the initial drying stage are avoided as occurs in the first pass of a three pass dryer.
U.S. Pat. No. 4,193,208 illustrates a single pass dryer having inwardly extending internal flighting within the drum which caused the material conveyed through the dryer to be lifted up and then dropped back into the hot gas stream, rather than simply resting at the bottom of the drum as it was rotated. The secondary flighting in the central part of the drum was provided to enhance heat exchange between the hot gases directed through the drum and the product to be dried. In order to prevent hot gases from being blown directly through the dryer from one end to the other, single pass dryers have included transverse plates in the drum to obstruct the flow of hot gases therethrough. The net result of such constructions was to decrease the capacity of the dryer while at the same time interfering with uniform temperature control and preventing maintenance of constant material flow rates through the dryer.
U.S. Pat. No. 5,157,849 illustrates and describes an improved single pass dryer having circumferentially spaced, inwardly directed, product conveying and showering conductive and convective heat transfer flights extending inwardly toward the center of the drum where the total surface area of the flights is at least as about as large as the total heat transfer surfaces of the products to be dried at maximum throughput capacity. The flighting design of the ""849 patent leaves a flight-free central showering zone of a size to permit heat exchange and conveyance of material along the length of the dryer at a predetermined rate, and establishes a specific range of diameter ratio between the diameter of the drum and the diameter of the internal cylindrical flight-free central product showering zone.
The present invention provides an improved single pass drum dryer exhibiting enhanced drying efficiencies while retaining the cost and operational advantages of a single pass dryer, as compared with a three pass unit. Broadly speaking, the drum dryer of the invention includes an elongated, hollow drum having a moist product inlet and a spaced dried product outlet, with a drying chamber between the inlet and the outlet. Flighting is provided within the drum which effectively separates the drying chamber into a plurality of drying stages, including a first stage adjacent the inlet, a final stage adjacent the outlet, and at least one intermediate stage between the first and final stages. The intermediate stage includes a plurality of drying zones arranged in successive order, from a point proximal to the first stage and extending towards the final stage. Each of the zones is configured with internal flighting having heat transfer surfaces that define a predetermined ratio calculated by dividing the total heat transferring surface area within the zone by the volume of the zone. The flighting is arranged so that the heat transfer ratio progressively increases from the first to the final zone within the intermediate stage. In preferred practice, one of the zones proximal to the first stage has a heat transfer ratio of from about 1.5-2.5 ftxe2x88x921, while another of the zones closer to the final stage has a heat transfer ratio of from about 2.75-3.75 ftxe2x88x921.
The preferred design of dryers in accordance with the invention is that the intermediate stage zones are arranged in contiguous relationship, with the first zone being contiguous with the first stage and the last zone being contiguous with the final dryer stage. The number of intermediate stage zones is variable, but usually ranges from 2-8, with four zones being most preferred. In the case of a four zone intermediate stage dryer, the first zone has a heat transfer ratio of from about 1.5-2.5 ftxe2x88x921, the second zone has a heat transfer ratio of from about 1.75-2.75 ftxe2x88x921, the third zone has a heat transfer ratio of from about 2.25-3.25 ftxe2x88x921, and the fourth zone has a heat transfer ratio of from about 2.75-3.75 ftxe2x88x921.
The intermediate stage zones are advantageously equipped with heat transfer flighting which presents a series of inwardly extending, circumferentially spaced apart metallic heat transfer panels, with the number of panels in each of the zones increasing from the first to the last zone. In practice, the panels are supported on corresponding strut elements coupled to the inner surface of the drum; these strut elements support L- and Z-shaped members which cooperatively define the individual panels.
The final stage of the preferred dryer has a heat transfer ratio smaller than the heat transfer ratio of any of the intermediate stage zones, and is preferably designed as a curing chamber of the type described in U.S. Pat. No. 5,157,849, incorporated by reference herein.
In operation, initially moist product (e.g., distillers grain, bakery wastes, alfalfa, peat moss, wood materials or similar particulates) is introduced into the dryer inlet along with heated air during rotation of the drum. Typically, the moisture content of the incoming product would range from about 30-70% by weight, while the inlet air temperature would be from about 600-1800xc2x0 F.; where distillers grain products are being dried, the temperature would be normally be from about 550-700xc2x0 F. Air flow rates through the dryer would commonly range from about 60,000 CFM to about 180,000 CFM.
As the product is advanced along the length of the drum by virtue of drum rotation and passage of air therethrough, it is progressively dried. At the same time, the air temperature decreases along the drum length. In the distillers grain example, the air would have a temperature of around 450xc2x0 F. as it enters the intermediate stage, and a temperature of about 225-250 xc2x0 F. into the third stage. The exiting air would have a temperature on the order of 190xc2x0 F. In the first stage, product drying is primarily from convective heat transfer, while in the second stage a combination of convection and conductive drying is carried out in the final stage, almost all of the product drying is accomplished by conduction.
The progressively increasing lighting density within the intermediate stage drying zones is important in obtaining high drying efficiency. First of all, as the product loses moisture during passage through the drum it becomes lighter, and more conductive heat transfer surface area is required to continue the drying process as the product lightens. However, the lighter product will increase the pneumatic influence on the flow of the product. Thus, product travel is reduced for a given air flow through the dryer, so that air flow velocities can be increased while still maintaining the desired air discharge temperatures.